Well, assuming you're talking about optimum cruise altitude for range ....
The higher you go, the more TAS you get for a given CAS, hence the more real ground you cover for a given amount of thrust. So, higher is better - ONCE you get up there.
BUT
It costs time, and fuel to get up there. And the heavier you are, the more time and fuel it takes. So eventually the penalty of climbing outweighs the gain in the cruise, and therefore the optimum altitude is lower for a heavier aircraft.
Pretty simplistic, and one of the perf experts can give you chapter and verse no doubt.